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A026557
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a(n) = T(n, n-4), T given by A026552. Also a(n) = number of integer strings s(0),...,s(n) counted by T, such that s(n)=4.
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18
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1, 3, 12, 28, 93, 201, 631, 1316, 4037, 8259, 25052, 50680, 152782, 306958, 921982, 1844304, 5526849, 11024331, 32987492, 65675764, 196323853, 390374193, 1166171943, 2316881892, 6918228187, 13737041045, 41007165500
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OFFSET
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4,2
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LINKS
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FORMULA
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MATHEMATICA
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T[n_, k_]:= T[n, k]= If[k==0 || k==2*n, 1, If[k==1 || k==2*n-1, Floor[(n+2)/2], If[EvenQ[n], T[n-1, k-2] + T[n-1, k] + T[n-1, k-1], T[n-1, k-2] + T[n-1, k]]]]; (* T=A026552 *)
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PROG
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(Sage)
@CachedFunction
if (k==0 or k==2*n): return 1
elif (k==1 or k==2*n-1): return (n+2)//2
elif (n%2==0): return T(n-1, k) + T(n-1, k-1) + T(n-1, k-2)
else: return T(n-1, k) + T(n-1, k-2)
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CROSSREFS
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Cf. A026552, A026553, A026554, A026555, A026556, A026558, A026559, A026560, A026563, A026566, A026567, A027272, A027273, A027274, A027275, A027276.
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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